In Project 4, Genetic Variation in Support of Murine Serotonergic Phenotypes, Sanders-Bush and colleagues take advantage of genetic and phenotypic variation present in recombinant inbred (Rl) strains of mice to illuminate 5HT gene networks that provide for dynamic flexibility in 5HT signaling, behavior and drug responses. Two critical proteins affecting serotonergic signaling capacity are brain tryptophan hydroxylase (TPH2), the rate-limiting enzyme for the production of brain 5HT, and the serotonin transporter (SERT), responsible for presynaptic reuptake of exocytosed 5HT. In the C57BL/6J and DBA/2J standard inbred strains of mice, a single nucleotide polymorphism is present in both the gene for brain TPH (Tph2) and the gene for SERT (Slc6a4). Evidence supports altered function of the proteins encoded by these allelic variants, motivating further in vivo study of their combined effects. We have selected 40 BXD recombinant inbred lines to represent, on a randomized C57BL/6J and DBA/2J genetic background, every allelic combination of the functional SNPs in Tph2and Slc6a4. This balanced factorial design supports the estimation of additive or epistatic modes of action of these SNPs on 5HT linked endophenotypes (Aim 1) and behaviors (Aim 2), and serves to seed and constrain more comprehensive modeling of the dorsal raphe transcriptome by systems genetics approaches (Aim 3). Aim 1 asks if Tph2 and Slc6a4 genotype alters a suite of 5HT measures, including levels of 5HT and function of SERT, 5HT turnover and extracellular levels of 5HT, and levels and function of 5HT1A, 5HT2A, and 5HT2C receptors. Aim 2 asks if Tph2 and Slc6a4 genotype predicts multiple behaviors for anxiety and depression and SSRI response, behaviors thought to model autism traits, and circadian behavior. Aim 3 determines the chronic transcriptome response to functional SNPs in Tph2 and Slc6a4, and further will describe internally validated gene network models correlated with these SNPs and the phenotypes described by Aims 1 and 2. The presence of functional SNPs in genes encoding critical proteins affecting 5HT signaling capacity, and the combined expertise of Conte colleagues, provides n unprecedented opportunity to integrate our in-depth understanding of the underlying olecular architecture of 5HT signaling into an exploration of networks and systems that control 5HT assembly and function in a complex genetic background.